To an increasing extent, cooking appliances are also being equipped with a steam generating system in order to improve the results of the cooking process with the help of the humid air thereby ensuing in the cooking chamber. Such cooking appliances are, for example, combination steamers, baking-ovens, steaming appliances or hot-air ovens. They serve, in particular, for the purposes of preparing food for consumption. It is preferred hereby that the steam be produced by injecting water directly into the cooking chamber. To this end, water is supplied to the cooking chamber and distributed therein in different forms and is thereby evaporated by the hot environment.
In accordance with the concepts known from DE 101 58 425 C1, DE 39 09 283 C2 or EP 0 233 535 B1 for example, water supply pipes are employed for this purpose, said pipes conveying the water to a point close to the hub of the fan wheel in a rotary blower. Due to the fact that the fan wheel is rotating about its hub, the water is, for example, fed from the hub to the impeller blades of the fan by centrifugal force and there, the water is decomposed into drops of as small a size as possible and these then evaporate in the hot atmosphere of the cooking chamber. The rotation of the fan wheel thus leads to the atomisation process.
It is also desirable for the rate of flow of the water being supplied to be regulated or controlled. To this end, DE 39 09 283 C2 describes a hot-air oven in which the water supply is equipped with a solenoid valve. Moreover, provision is made for the temperature in the exhaust air flue to be measured. The solenoid valve is thus operated for controlling the supply of water in dependence on the temperature-measured in the exhaust air flue. However, if the water flow is switched off completely, undersaturation and thus impairment of the cooked item can occur due to losses or absorption of the steam by the cooked item.
In order to avoid this effect, the users try to get round this disconnection process by not using the system.
Furthermore, a proposal is known from DE 202 00 618 U1 which enables the quantity of water to be reduced, this thereby avoiding the dangers ensuing if the flow of water is completely switched off. Here, the water supply is equipped with a controllable water dosage means for dosing the quantity of water flowing through the arrangement. The result of this is that the rate of flow of the water is no longer maintained at approximately the same level but is reduced after reaching saturation. This reduction enables a substantial saving of energy to be achieved during the cooking process.
As in DE 39 09 283 C2, adjustable solenoid valves are also employed in other cases in order to switch on or switch off the flow of water to the cooking chamber. These valves are usually regulated by an electronic control system. The rate of flow is comparatively small and amounts in practice, to between 3 and 25 litres per hour, corresponding approximately to 1 cm3/s to 7 cm3/s in dependence on the size of the oven.
However, there are large fluctuations of pressure in the public water supply network and the pressure varies very widely from place to place. In practice, the pressure in the pipes may be between 0.2 and 1.0 MPa, i.e. between about 2 and 10 bar.
However, a constant rate of flow would be a prerequisite for maintaining a uniform and reproducible quantity of steam and thus obtaining reproducible results from the cooking process. If the rate of flow for generating the steam varies, this has an effect on the temperature in the cooking chamber since substantial quantities of energy must be expended in order to evaporate the amount of water that has been introduced. Too large a variation in the rate of flow means a large change in value of a disturbance variable which rapidly leads to excessive demands on the temperature regulator. A deviation from the preferred temperature, particularly in the steam mode of operation, then leads to the results of the cooking process being unsatisfactory. In the event that the amount of water supplied is too small, the amount of steam produced may be so low that this will have negative effects on the result of the cooking process.
In the present state of the art for example, mechanical water pressure regulators are employed in order to maintain the variable water input pressure at a constant initial value. However, these pressure regulators have the disadvantage that the initially set pressure values alter during the operational period and, due to their method of construction, they exhibit inertia which prevents rapid readjustment of the pressure in the face of a rapidly changing input pressure.
Furthermore, the regulators and the valves being utilised need to be initially adjusted.
Moreover, a disadvantage of the state of the art is that the initially set rate of flow can only be altered to a limited extent by the electronic control system. The rate of flow can in fact be reduced by applying clock pulses to the solenoid valve, but it cannot be increased without further measures.